Earlier quoted context omitted.
The US Federal government uses the following definition: the term "terrorism" means premeditated, politically motivated violence perpetrated against noncombatant targets by subnational groups or clandestine agents https://uscode.house.gov/view.xhtml?req=(title:22%20section:...
And yet the same Federal government called the attack against USS Cole "terrorism" and its perpetrators "terrorists", and implemented a range of "anti-terrorism" measures in the Navy after that. And it was a military target, and all the victims were military. So really, terrorism is what people say is terrorism.
> The extensive FBI investigation ultimately determined that members of the al Qaeda terrorist network planned and carried out the bombing.
[...]
> By the end of 2000, Yemeni authorities had arrested several suspects, including Jamal Muhammad Ahmad Al-Badawi and Fahad Muhammad Ahmad Al-Quso
(Source: https://www.fbi.gov/history/famous-cases/uss-cole-bombing)
You don't generally call in the FBI to "investigate" a military attack, or "arrest suspects".
If the nation of Yemen had immediately claimed responsibly for the exact same attack and declared war, it wouldn't have been considered terrorism, and rightfully so.